The present invention relates generally to polymeric foam processing, and more particularly, to polyethylene foams and methods of their production
Polymeric foams include a plurality of voids, also called cells, in a polymer matrix. By replacing solid plastic with voids, polymeric foams use less raw material than solid plastics for a given volume. Thus, by using polymeric foams in many applications instead of solid plastics, material costs are reduced.
Microcellular foams generally, have smaller cell sizes and higher cell densities than conventional polymeric foams. Typically, microcellular foams are defined as having average cell sizes of less than 100 microns and a cell density of greater than 106 cells/cm3 of solid plastic. In a typical continuous process for forming microcellular foam (e.g. extrusion), the pressure on a single-phase solution of blowing agent and polymer is rapidly dropped to nucleate the cells. The nucleation rate must be high enough to form the microcellular structure.
Several patents describe aspects of microcellular materials and microcellular processes. U.S. Pat. No. 4,473,665 (Martini-Vvedensky, et al.; Sep. 25, 1984) describes a process for making foamed polymer having cells less than about 100 microns in diameter. In a technique of Martini-Vvedensky, et al., a material precursor is saturated with a blowing agent, the material is placed under high pressure, and the pressure is rapidly dropped to nucleate the blowing agent and to allow the formation of cells. The material then is frozen rapidly to maintain a desired distribution of microcells.
U.S. Pat. No. 5,158,986 (Cha, et al.; Oct. 27, 1992) describes formation of microcellular polymeric material using a supercritical fluid as a blowing agent. In a batch process of Cha, et al., a plastic article is submerged at pressure in supercritical fluid for a period of time, and then quickly returned to ambient conditions creating a solubility change and nucleation. In a continuous process, a polymeric sheet is extruded, and then can be run through rollers in a container of supercritical fluid at high pressure, and then exposed quickly to ambient conditions. In another continuous process, a supercritical fluid-saturated molten polymeric stream is established. The polymeric stream is rapidly heated, and the resulting thermodynamic instability (solubility change) creates sites of nucleation, while the system is maintained under pressure preventing significant growth of cells. The material then is injected into a mold cavity where pressure is reduced and cells are allowed to grow.
International patent publication no. WO 98/08667 (Burnham et al.) provides methods and systems for producing microcellular material, and microcellular articles. In one method of Burnham et al., a fluid, single phase solution of a precursor of foamed polymeric material and a blowing agent is continuously nucleated by dividing the stream into separate portions and separately nucleating each of the separate portions. The divided streams can be recombined into a single stream of nucleated, fluid polymeric material. The recombined stream may be shaped into a desired form, for example, by a shaping die. Burnham et al. also describe a die for making advantageously thick microcellular articles, that includes a multiple pathway nucleation section. Other methods describe the fabrication of very thin microcellular products, as well. In particular, a method for continuously extruding microcellular material onto a wire, resulting in very thin essentially closed cell microcellular insulating coating secured to the wire, is provided. In some of the methods, pressure drop rate is an important feature and techniques to control this and other parameters are described.
High-density polyethylene (HDPE) has traditionally been a difficult material to process as a foam. This, in part, arises from the low melt strength of HDPE. Processes that employ chemical blowing agents have been developed to produce foams from high-density polyethylene. Additionally, HDPE foams have been produced by batch processes (see, for example, U.S. Pat. No. 5,158,986). However, the applicants are unaware of extruded or injection molded foams from HDPE produced without the use of chemical blowing agents or without the addition of another polymeric component, for example low density polyethylene (HDPE) or linear low density polyethylene (HDPE). It is an object of the invention, therefore, to provide a non-batch process for producing HDPE foam using a physical blowing agent.
The invention provides HDPE foams and processes for their production. The HDPE foams can be formed in extrusion, injection molding, or blow molding processes using physical blowing agents, and thus the foams are essentially free of residual chemical blowing agent and reaction-by-products of chemical blowing agent. The HDPE foams can be produced over a broad density range and formed into a variety of articles.
In one aspect, the invention provides a foam article that includes a matrix of polymeric material including a plurality of cells. The polymeric material consists essentially of high-density polyethylene and is essentially free of residual chemical blowing agent and reaction-by-products of chemical blowing agent. The article has a shape essentially identical to that of a continuous extrudate or the interior of a mold.
In another aspect, the invention provides a method of forming a foam article. The method includes the step of conveying polymeric material in a downstream direction in a polymer processing apparatus. The polymeric material consists essentially of high-density polyethylene. The method further includes the steps of introducing a physical blowing agent into the polymeric material in the polymer processing apparatus and forming a foam article from the polymeric material.
Among other advantages, the invention provides processes for producing HDPE foams that use physical blowing agents instead of chemical blowing agents. Physical blowing agents are often less expensive than chemical blowing agents. Finally, processes using physical blowing agents are more efficient and reliable, not having to depend on a chemical reaction to determine the amount of blowing agent released during the foaming process.
Furthermore, the invention provides an HDPE foam containing low amounts of, or essentially free of residual chemical blowing agent and reaction-by-products of chemical blowing agents. In some cases, the presence of residual chemical blowing agents and reaction-by-products of chemical blowing agents in a material is detrimental and can restrict its use. The HDPE foams, in accordance to the invention, are advantageously suitable for applications, such as food packaging, and are more easily recycled without adverse effects.
In another aspect the invention provides specific die designs useful for making high quality polymeric foams, and particularly microcellular HDPE foams. The die can be provided as part of a system of extrusion. The die includes a nucleating pathway that decreases in cross-section in a downstream direction with an included angle of greater than 4xc2x0.
In another embodiment of this aspect of the invention a method is provided that involves introducing a single-phase solution of polymeric material and blowing agent into an inlet of a polymeric foaming die, nucleating the single-phase solution in a nucleating pathway that decreases in a downstream direction with an included angle of greater than 4xc2x0 to form a nucleated polymeric stream, and releasing the nucleated stream as a polymeric foam extrudate to form a nucleated polymeric stream.
In another embodiment a forming die according to this aspect of the invention includes a nucleating pathway constructed such that when a single-phase, non-nucleated solution of polymeric material and blowing agent is introduced into the die and conveyed through the die at a flow rate of about 100 pounds per hour, nucleation of the solution occurs to form a nucleated polymeric stream that is released from the die in a period of time of no more than about 0.002 second after nucleation.
In another embodiment of this aspect of the invention a method is provided that involves introducing a single-phase, non-nucleated solution of polymeric material and blowing agent into a polymer forming die. Within the die the solution is nucleated to form a nucleated polymeric stream. The stream is released as a polymeric microcellular extrudate from an outlet of the die in a period of time of no more than about 0.002 second after nucleation.
Other advantages, novel features, and objects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.